The proliferation of laptops and hand-held portable computers has produced a concomitant need for robust, reliable and high performance wireless network to maximize the mobility advantages of these wireless devices and increase the ease of construction and management of the wireless networks.
The IEEE 802.11 standard specifies the media access control (MAC) and physical (PHY) layers for wireless local area networks (WLANs). Generally, there are two variants of WLANs: the infrastructure-based type and the ad-hoc type wireless network. In the infrastructure-based network, the communication typically takes place only between the wireless nodes called stations (STAs) and an access point (AP), thus the wireless nodes (STAs) can exchange data via the access point (AP). In the ad-hoc type network, the communication takes place directly between the wireless nodes (STAS) unlike the way of the former network.
The wireless nodes (STAS) within the same radio coverage are known as a basic service set (BSS) which may be established and identified by a main wireless device as a centralized decision maker. For example, in the infrastructure-based network an access point (AP) will establish a BSS according to the IEEE 802.11 standard, while in an ad-hoc type wireless network, a station identifying the BSS will take a role as the centralized decision maker for other stations joining the BSS.
When two adjacent BSSs are located close to each other and operate at the same or adjacent channel, which are referred to as overlapping BSSs, it is difficult to support the required quality-of-service (QoS) due to the possible mutual interference between the overlapping BSSs. In addition, other co-located systems near a particular access point may cause communication interference. It is not always possible to avoid interference by carefully planning channel allocations to BSSs before the WLAN deployment, especially in the home/office environment where other WLAN devices are operating independently in the vicinity.
Some dynamic frequency selection (DFS) schemes are known to be able to solve the problem and incorporate with IEEE 802.11 standard. In most of existing DFS schemes, the quality of each available channel is measured first by the stations and then report to the access point. The channel with least interference and best quality will be selected as a new channel for the BSS to switch into. In a known dynamic frequency selection mechanism, the operational channel of the WLAN is selected dynamically according to channel quality parameters of Received Signal Strength Indication (RSSI) and Clear Channel Assessment (CCA) busy periods obtained by the AP during channels measurement. However, since these known DFS schemes all rely on the communication between the stations and the access point, they introduce some changes to the current 802.11 specifications, which may bring to a compatibility problem between wireless devices of different manufacturers.
In addition, the conventional DFS schemes can not handle a case that a plurality of BSSs are automatically selecting channel concurrently, for example, when two APs are powered on simultaneously. The access points that are initiating channel selection concurrently will dirty the detecting result of existing operational BSSs and interfere with the decision of the selection of a new channel. In this case, the APs may select the same channel because they detect the same wireless environment.
Accordingly, there is a need for an improvement of dynamic frequency selection scheme which not only meets IEEE 802.11 standard, but also enables selecting of frequency channels appropriately even when a plurality of access points are automatically selecting channel concurrently.